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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a global position system (GPS)
receiver, and more particularly to a portable GPS receiver having an
intelligent power supply controller capable of reducing the battery power
consumption without position accuracy degradation.
2. Description of the Prior Art
GPS is a positioning and navigation system which receives signals from a
plurality of satellites for determining a two or three dimensional
position of the receiver. This positioning system is capable of performing
a position determination over the entire surface of the globe by receiving
signals from a subset of twenty four satellites. These satellites operate
on six orbits about 20,200 KM above the earth with each orbit
accommodating four satellites. In receiving the signals from these
satellites, a GPS receiver periodically computes the latitude, longitude,
altitude and time on a real time basis.
To determine a three-dimensional position, the signals from four satellites
are required while to make a determination of a two-dimensional position,
the signals from three satellites are sufficient. Typical examples of the
former are satellites and airplanes and those of the later are ships and
cars. However, a three-dimensional position is often required if an
automobile is travelling in a mountain region where there are great
elevation variations in travelling a short distance.
The GPS system was originally developed for military use. It was then made
available for civilian applications including navigation systems for
ships, aircraft and automobiles. In the past few years, portable,
hand-held GPS receivers have also become publicly available. These
convenient portable receivers were made possible, in part, as a result of
the miniaturization of electronic devices which continues to reduce the
size, weight and power consumption of the electronic components. The
portability of a GPS receiver however is often limited by the size and
weight of the batteries providing power to the receiver. To sustain
prolong periods of operation, a heavy and bulky battery system is
required. On the other hand, use of light-weight small batteries require
either frequent re-charge or replacement thus making the operation of such
a hand-held receiver more expensive and less convenient.
Many U.S. patents disclose power saving methods for digital signal
receiving or paging systems. Basically, a receiver or transmitter of these
systems is maintained in an "idle" state with very low or complete off
power states. Either a hardware or a software system is provided to
monitor when incoming signals are received. The receiving system is
activated when incoming signals are detected. Then the power is
automatically turned off after reception of the signals is complete. Even
though this general concept of power saving is widely known, the
implemented methods however are not of practical use to a GPS receiver.
Unlike the general digital receivers, paging or remote telephone systems,
a GPS receiver maintains a table listing all the visible satellites and
their positions. A GPS receiver must receive almanac signals from these
satellites every twelve hours to compute the satellite positions with
moderate accuracy. A more accurate satellite position calculation must
also be performed by receiving ephemeris data from the satellites every
hour. The collection of almanac data takes about twelve minutes and the
collection of ephemeris takes about thirty seconds. A GPS receiver is not
totally "idle" even between the signal reckoning times. Continuous status
checking of satellites and the data receiving channels must also be
performed. Depending on the results of the status checking, a GPS receiver
then determines a schedule to sequentially activate each signal receiving
channel. Therefore, the method of maintaining an idle state and passively
waiting before the arrival of a signal to save battery power as disclosed
in the U.S. patents for the general digital signal receiving systems are
not useful for reducing the power consumption in a GPS receiver.
Several commercially available GPS receivers are on the market which
utilize a set of six AA size alkaline batteries and various means are
provided for the receiver operator to save power in preserving the battery
life. In SportNav, a Loran C receiver system, a twenty-five hour period of
operation is estimated with six AA alkaline batteries. The user is
provided the option of a backup battery pack so that the batteries can be
quickly replaced. No specific power saving mechanism is implemented in
this product.
Another product with the model name PRONAV GPS 100 uses six disposable
alkaline batteries and a rechargeable battery pack. It also allows the use
of an external power source to provide continuous navigation updates. GPS
100 has a "Battery Saver Mode" operable on a pack of alkaline batteries
for fourteen hours and a "QuickFix Mode" which automatically completes
four position fixes per hour and allows the receiver to operate for longer
periods of time with six alkaline batteries. Under most dynamic
circumstances, use of QuickFix to obtain four position fixes per hour is
not satisfactory. The usefulness of PRONAV GPS 100 is limited because the
length of battery life is likely to be greatly shortened when the limited
operations allowable under the "Battery Saver Mode" or "QickFix Mode" are
not sufficient to satisfy the position accuracy requirements unless there
is external power source readily available.
Another hand-held GPS receiver, the Magellan NAV 1000, is powered by six AA
alkaline batteries. For the purpose of reducing battery power consumption
and extending the life of the batteries, a PowerSaveR mode is provided
under which the receiver can be manually turned on to compute a position
fix. After the position fix is stored as the last fix, the receiver then
turns itself off. The receiver can also operate continuously and
automatically revert to PowerSaveR mode when a `battery low` condition is
detected. NAV 1000 also allows the unit to operate on an external power
source. It is instructed in the User's manual not to collect almanac
information in hand-held operation using the battery because of the
concern of the limited battery life. The usefulness of the hand-held GPS
receiver would probably be limited due to these limitations.
Therefore, the prior art hand-held portable GPS receivers operating on
battery power are typically useful for a very limited period of time if
operated continuously. Except where an external power source is readily
available, position fix computations on the order of once per second or
even once per minute in order to minimize dead reckoning errors would not
be possible. This greatly limits the application of hand-held GPS
receivers. When no external power source is available, a hand-held battery
operated GPS receiver has only limited usefulness due to the short battery
life.
SUMMARY OF THE PRESENT INVENTION
It is therefore an object of the present invention to provide a
battery-powered GPS receiver which can operate for a long period of time
while maintaining a sufficient calculation rate.
It is another object of the present invention to provide a battery-powered
GPS receiver which can continuously and dynamically respond to various
operational conditions to minimize battery power consumption.
It is a further object of the present invention to make use of the
intelligence of an executable program implemented on a read only memory
(ROM) to control the power supply to a GPS receiver.
Briefly, in a preferred embodiment, the present invention comprises a
global positioning system (GPS) receiver having radio frequency (RF)
circuitry to receive position signals from a plurality of satellites and
provide an intermediate frequency (IF) signal to a correlator circuit for
generating a pseudo range and a Doppler measurement for calculating a
position fix. The GPS receiver further has a power supply system having at
least one battery and an alternative external power connector for
connecting to an external DC power source. The GPS receiver further has a
microprocessor and a read only memory (ROM). The microprocessor is
electrically connected to the RF circuitry and the power supply system.
The ROM includes an executable program capable of automatically
controlling the power supply system to provide a plurality of power levels
ranging from zero to a maximum power level to the RF circuitry so that the
battery power consumption is minimized.
It is an advantage of the present invention that the executable program
residing in the ROM can continuously and dynamically respond to the
operation conditions by adjusting the schedules and power level provided
to the RF circuitry to minimize any unnecessary power consumption.
It is another advantage of the present invention that with the capability
of responding to operational conditions, the battery-powered GPS receiver
can be used in inaccessible rural areas and heterogeneous mountain
districts with long battery life without sacrificing the frequency of
position fix computations.
It is a further advantage of the present invention that the collection of
almanac and ephemeris data, and the acquisition and reacquisition of
satellites can be carried out on a substantially regular basis because of
the very efficient management of battery power during these operation
cycles.
It is a further advantage of the present invention that a GPS receiver user
has the option to input several operational constants depending on the
user specific operations whereby the battery power can be optimally used
for the specific operation the user intends to apply.
These and other objects and advantages of the present invention will no
doubt become obvious to those of ordinary skill in the art after having
read the following detailed description of the preferred embodiment which
is illustrated in the various drawing figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of a GPS receiver according to the
present invention;
FIG. 2 is a flow chart diagram of the main receiver loop logic of the
receiver of FIG. 1;
FIG. 3 is a flow chart diagram of the power saving module Power.sub.--
Smart.sub.-- Mode of the loop logic of FIG. 2;
FIG. 4 is a flow chart diagram of the shut down module which conducts
several status checks and performs acquisitions or reacquisitions before
turning off the power to the RF circuitry of the receiver of FIG. 1;
FIG. 5 illustrates the acquisition time cycles implemented to conserve
battery power;
FIG. 6 is a flow chart diagram of the TURN.sub.-- OFF.sub.-- RF module of
the receiver of FIG. 1; and
FIG. 7 is a flow chart diagram of the TURN.sub.-- ON.sub.-- RF module of
the receiver of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 illustrates a GPS receiver 10 having an antenna 12, a radio
frequency (RF) circuitry 14, a power supply 16, a power supply controller
18, an application specific integrated circuit (ASIC) 22, a random access
memory (RAM) 23, a microprocessor 24, and a read only memory (ROM) 26.
Antenna 12 receives navigation data signals from a plurality of satellites
32. A list of visible satellites is kept in the random access memory (RAM)
23 accessible to microprocessor 24 and ROM 26. Based on the current
estimated position of the GPS receiver 10, the azimuth and the elevation
of each satellite 32 in the constellation are computed. If the elevation
angle of a satellite is positive and is greater than a mask angle, e.g.,
ten degrees, the satellite is considered visible. When a satellite first
becomes visible, it is first included in the "to be acquired" list and an
acquisition operation is executed to obtain the code phase and frequency
of the satellite signals and collect the satellite's ephemeris. The
visible satellite is added to the tracking list and maintained as under
the "tracking state" when the signals received from the satellite are
capable of generating measurements. Another situation is that a satellite
which has been in the tracking state suddenly is lost because the signals
are blocked by trees, tunnels, or buildings or because of excessive
vehicle dynamics. The satellite is categorized as being in the "lost
state". The executable program, residing in ROM 26, constantly checks the
states of the satellites 32 and sets a flag for each satellite indicative
of the current tracking state.
After the signals are received from the satellites 32, RF circuitry 14
first amplifies and then down converts the received RF signals to
intermediate frequency (IF) signals. ASIC 22 includes a clock, data
filters, demodulation circuits and integrators to simultaneously process
through multiple channels the signals received from multiple satellites
32. ASIC 22 receives the IF signals from RF circuitry 14 and generates the
pseudo-range (code phase) and the Doppler (frequency) measurements. These
measurements are then used by microprocessor 24 for position fix
computations.
With the advance of modern electronics, all electronic components are made
smaller in size and meanwhile capable of performing many intelligent
functions with less power requirements. However, the battery life of a
hand-held portable GPS receiver is still frequently limited by the power
requirements of the RF circuitry 14. In order to reduce power consumption
of the RF circuitry 14 and to prolong the battery life of GPS receiver 10,
power supply controller 18 is under the control of an executable program
residing in the ROM 26 to automatically adjust power level input to RF
circuitry 14 to efficiently utilize the electric power provided by power
supply 16.
FIG. 2 is a flow chart diagram showing the logic sequence performed by the
executable program residing in ROM 26 to control power supply 16. The ROM
executable program includes a MAIN.sub.-- RCVR.sub.-- LOGIC module 40
which is initiated when receiver 10 is turned on by a step 44. With a step
46, a first check is made to determine if power supply 16 is connected to
an external power source. Step 48 determines if power supply 16 is
connected to an external power source, and if so, the power saving module
POWER.sub.-- SMART.sub.-- MODE is not executed. If power supply 16 is not
connected to an external power source, an operator of the GPS receiver is
allowed an option to bypass the execution of the power saving program. A
check is make to determine if there is an user override command to bypass
the execution of the power saving module in a step 52. The power saving
module POWER.sub.-- SMART.sub.-- MODE is invoked in a step 54 when power
supply 16 is not connected to an external power source and there is no
user override command to bypass POWER.sub.-- SMART.sub.-- MODE power
saving module.
MAIN.sub.-- RCVR.sub.-- LOOP 40 is executed on a periodical basis such as
once per every second to compute and update the position fix and to
perform all other general functions of the receiver. The rate of position
fix computations is basically fixed. A lower rate of position fix may be
implemented when there is a need to save the battery power as determined
by the logic below. MAIN.sub.-- RCVR.sub.-- LOOP 40 also allocates the
visible satellites to the physical tracking channels and schedules the
measurements to be taken for each satellite on the tracking list.
FIG. 3 shows the steps executed by a power saving module POWER.sub.--
SMART.sub.-- MODE 60 within the program of ROM 26. In a step 62, it first
determines the on/off status of RF circuitry 14. If the RF circuitry 14
has the power turned off, the time that the power is off is first computed
in a step 64 and the time off is compared to the length of time that the
power is scheduled to be off, i.e., Toff in a step 66. If the time off is
still within the time window of scheduled time off, the execution of
POWER.sub.-- SMART.sub.-- MODE 60 is finished for this pass and returned
to the beginning of MAIN.sub.-- RCVR.sub.-- LOOP 40. Otherwise a
subroutine TURN.sub.-- ON.sub.-- RF is called in a step 72 before
returning in a step 74. On the other hand, if the RF power is on, in a
step 76 an enquiry is made to determine if time is within the window of an
RF burst, i.e., RF.sub.-- BURST=true, and if POWER.sub.-- SMART.sub.--
MODE is true. A normal continuous power mode with position fix computation
is executed in a step 78 proceeded with calling a shutdown logic module in
a step 82. If RF burst=true and POWER.sub.-- SMART.sub.-- MODE=true as
determined in step 76, the time of the RF power on is calculated in a step
84 and compared with the scheduled RF power on time window, i.e., Ton in a
step 86. It loops back to continue the wait if the calculated time on is
within the time window in a step 88, otherwise the RF burst is completed.
A test is made to determine how many good measurements, i.e., Ng, and how
many missed measurements ,i.e. Nm, are processed by use of the signals
received from the satellites in a step 92. When there are sufficient good
measurements to compute a three-dimensional (3-D) position fix, including
the conditions that 1) there are enough good measurements, Ng, to compute
a 3-D position fix in step 96, or 2) there is calibrated altimeter
available in a step 98 and a 3-D fix is available using ALTIMETER and GPS
in a step 102, or 3) there are no missed measurements, i.e. Nm=0 in step
94, then a shut down logic is called in a step 104 to turn the RF power
off and compute a position fix in a step 106.
Even if there are not sufficient measurements to compute a 3-D position but
if the vertical motion is small and two-dimensional (2-D) position
computation is achievable as determined in a step 108, again a shut down
logic module is called in the step 104 and a 2-D position fix is computed
in the step 106. If computation of either 3-D or 2-D position is not
achievable, then a check is first made to determine if VERICAL.sub.--
MOTION is true (step 109). A shutdown logic module is called if
VERICAL.sub.-- MOTION is not true, otherwise a sequence of status checks
are made to determine whether another RF burst will be performed (step
116) and the RF power will continue to stay on for one more RF burst. In
step 110, it is first determined if a three-dimensional fix is performed
during the last RF burst and if there are three good measurements and
there are at least four visible space vehicles. After satisfying the test
conditions in step 110, one more RF burst is executed (step 116),
otherwise further tests are performed in step 112 to determine if another
RF burst should be executed, based on the test results from step 112,
which determines whether a two-dimensional fix is likely to be successful.
Only one more RF burst is allowed because QUICK.sub.-- BURST is set to
true in step 116 and step 114 checks the value of QUICK.sub.-- BURST
allowing the RF burst to be executed only once. The purpose of this logic
is to attempt to at least obtain a two-dimensional fix even if there is
vertical motion. The two-dimensional position fixes may not be
sufficiently accurate but they can be useful for a GPS receiver to search
for the satellites.
The length of each RF burst depends on the number of channels, the number
of satellites and the length of time required for each satellite. For
example, if GPS receiver 10 is a three-channel receiver and the time
required for each satellite is 0.5 second, for a group of seven satellites
an RF burst of 1.5 seconds,i.e. three times 0.5 seconds, is required,
where three is the number of RF cycles needed to handle seven satellites
with three channels. The length of the RF burst is thus designed to be
sufficient to obtain a measurement from each satellite on the tracking
list.
FIG. 4 shows the logic of a SHUT.sub.-- DOWN Module 120 of the program of
the ROM 26. When SHUT.sub.-- DOWN module 120 is called, it first sets
QUICK.sub.-- CYCLE to a value of false (step 121) and checks whether the
GPS receiver 10 knows the time in a step 122. If so, it checks if there is
a position fix in a step 124, and if yes, if it is time to collect more
almanac or ephemerals data in a step 126. The RF power is kept on if
either of the test results for the first two enquiries, i.e. steps 122 or
124, are negative or the test result of the third enquiry, i.e. step 126,
is positive. The age of the GPS almanac is checked periodically so that it
is generally no more than twelve hours old. A flag is set if a new almanac
is needed or if no almanac is available. Before the RF power is turned
off, it is further tested if any visible space vehicles (SVS) which have
not been acquired in a step 128, or if any SVS have been lost and whether
a three-dimensional fix having a PDOP less than six (step 132) is not
available with the current tracking list. If either of the above tests,
i.e. steps 128 or 132, are true, a satellite acquisition cycle begins in a
step 134 or 136 and a flag TRYING.sub.-- TO.sub.-- ACQ is set to 1, i.e.
true, otherwise it is set to zero, i.e. false in a step 138. The purpose
of step 132 is to save power when there are lost satellites but a good fix
is available anyway. TURN.sub.-- RF.sub.-- OFF is then called to save
power without trying to acquire the lost satellite. Otherwise,the
acquisition is attempted for a maximum number of times, i.e. ON.sub.--
CNT.sub.-- MAX in step 142 and for a maximum number of cycles, i.e.
CYCLE.sub.-- MAX in step 144 then the cycle is reset to zero in a step 146
and the RF power is turned off in a step 148.
In order to acquire a new satellite, a large range of frequency and code
phases must be searched to find the satellite signals, thus it takes a
longer duration to acquire new satellites with RF burst. Therefore,
initial acquisition will require the RF circuitry 14 to be on for a period
of time which is adequate to complete the search and the search must also
be made consecutively in time. It is normal that new satellites added to
the tracking list will not be acquired, for as long as a few minutes, when
the elevation mask is low because of blocking by trees or buildings. In
order to conserve battery power, the acquisition is only attempted
periodically with the RF on for an adequate time to properly search for
the satellite. The power provided to RF circuitry 14 is turned on
continuously for a period of ACQ.sub.-- ON.sub.-- TIME and then turned off
until next RF ACQ.sub.-- CYCLE.sub.-- TIME is due (FIG. 5). A power saving
is achieved because the RF power is not kept on through the entire
acquisition time. A similar technique is used to acquire lost satellites.
Reacquisition is treated slightly different because there is typically
more information available about a lost satellite and it can be found in
shorter length of time. The on and off times are set differently. An
example of time limits for reacquisition is to maintain the power on for a
time period of ten seconds and a cycle time of thirty seconds, while for
initial acquisition the power is kept on for thirty seconds instead of ten
and a cycle time of one hundred and twenty seconds instead of thirty.
The acquisition and reacquisition operations are implemented in the
SHUT.sub.-- DOWN logic. Before the RF circuitry 14 is turned off, attempts
are made to acquire and reacquire the new or lost satellite. The
acquisition or reacquisition are repeated a fixed number of cycles before
proceeding to shutoff of the RF power. Since the SHUT.sub.-- DOWN logic is
called repeatedly after a fixed time interval, acquisition and
reacquisition will be attempted again if the previous operation fails.
Since for a hand-held GPS receiver 10, the most frequent situation is the
temporary loss of communication due to tree or building blockage, power
saving is achieved by not keeping the RF power for the entire period of
time when trying to reacquire the lost satellite.
When a SHUT.sub.-- DOWN is performed, the power provided to RF circuitry 14
is turned off for a brief period of time which depends on the desired
accuracy of the dead reckoning while the RF is off, the number of channels
of the receiver, the distribution of the satellites to each channel, and
the way that satellites are acquired. For a hand-held GPS receiver 10,
because the acceleration is generally low and the accuracy requirement is
typically at about 100 meters, a turn off time of four to five seconds is
generally acceptable.
An important factor for power savings as taught in this invention is that
the benefits of power savings are not disrupted when a satellite is lost
or not yet acquired. The acquisition is accomplished with the RF power
turned on only periodically. Meanwhile a test is made to determine if a
2-D solution is sufficient. No second attempt will be made if there is
enough data for a 2-D position fix. The RF power is turned off right after
the RF burst.
FIG. 6 illustrates the logic of TURN.sub.-- OFF.sub.-- RF module 150 of the
ROM 26. It first calculates the on time, i.e. Ton and off time Toff by
first determining the maximum number of space vehicles (SVS) on each of
the GPS receiver channels in a step 152 and Ton is the product of maximum
number of SVS and the time per measurement which is fixed constant in a
step 154. Toff is computed by subtracting the on time from the cycle time
in a step 156. The RCVR tracking loops are put in an idle state in a step
158 and a hardware command is issued to activate a switch to shut off
power to RF in a step 162 to save power. The state of RF power is set to
off in a step 164 and the time when RF power is turned off is set to the
current time in a step 166. The RF power will be turned on when a preset
scheduled time off is up (see steps 66 and 72).
Instead of setting the CYCLE.sub.-- TIME as a constant, TURN.sub.--
OFF.sub.-- RF logic 150 adaptively changes the CYCLE.sub.-- TIME as a
function of speed. This adaptive process is enabled by a user's setting
ADAPTIVE=TRUE. This is particularly useful when a GPS receiver is moving
at a high speed. It is to be noted that if a receiver is moving at a
constant velocity vector there is no dead reckoning (DR) error if even the
receiver is moving at high speed. However, if a turn is made at high
speed, a large DR error is generated. By making the CYCLE.sub.-- TIME a
function of speed, the magnitude of the DR error can be reduced. It is
first tested to determine if a user optional input, i.e., ADAPTIVE, is set
to be TRUE (step 156-1). If ADAPTIVE is TRUE and the the speed of the GPS
receiver is greater than a threshold speed S2, as determined by step
156-1, then the power to RF is kept on continuously and the execution of
TURN.sub.-- OFF.sub.-- RF logic 150 is terminated and returned to the
calling module (step 156-3). The RF power is kept on until the speed is
reduced below S2 or when the user turns off the adaptive process by
setting ADAPTIVE to FALSE. If the speed of the receiver is less than S2 as
determined by step 156-1, then the speed is compared to a first threshold
speed, i.e. S1 in step 156-4. If the speed is greater or equal to S1, then
a new cycle time which is half of the regular cycle time is used (step
156-5), otherwise a regular CYCLE.sub.-- TIME is kept unchanged and the
program proceeded with step 156.
FIG. 7 shows the logic operations of TURN.sub.-- ON.sub.-- RF module 170 of
the ROM 26. A hardware command is first issued to activate the switch to
turn on the RF power in a step 172. The RF tracking loop is set to a
reacquisition state, i.e. RE.sub.-- ACQ=1, in a step 174. The RF burst is
set to 1 (TRUE) to begin the RF burst cycle in a step 176 and the starting
RF on time, i.e. t-on, is set to the current time in a step 178. The
executable program residing in ROM 26 further computes a velocity vector
of the GPS receiver 10 to determine the number of satellites and therefore
position signals needed to compute a position fix, e.g., the number needed
to satisfy a position accuracy requirement. A determination is made of the
number of satellites that are providing position signals. The executable
program turns-off power to the RF circuitry for a predetermined duration
of time when the actual number of satellites equals or exceeds the
required number of satellites, in order to conserve battery power.
The executable program residing in ROM 26 preferably includes a routine for
determining if the actual number of satellites visible is less than the
number of satellites required for position signals, e.g., a two
dimensional position fix may be sufficient and available.
The executable program residing in ROM 26 stores a plurality of historical
position fix data and a list of the visible satellites. If the actual
number of satellites that position signals are being received from is less
than a required number, the history of position fix data and the list of
the visible satellites is used to determine if an additional RF burst
should be performed to add the available measurement data.
Although the present invention has been described in terms of the presently
preferred embodiment, it is to be understood that such disclosure is not
to be interpreted as limiting. Various alternations and modifications will
no doubt become apparent to those skilled in the art after reading the
above disclosure. Accordingly, it is intended that the appended claims be
interpreted as covering all alternations and modifications as fall within
the true spirit and scope of the invention.
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